At one time, a diagnostic feature of the Porifera was the presence of
spicules. As a result, certain fossil groups whose
organization was consistent with that of living sponges were not placed within the
phylum Porifera. In particular, groups with a solid calcareous skeleton such as the
Archaeocyatha,
chaetetids,
sphinctozoans,
stromatoporoids, and
receptaculids were problematic.
A great deal of insight into the phylogenetic affinities of these groups was gained with
the discovery of more than 15 extant species of sponges having a solid calcareous
skeleton. These species are diverse in form, and would be classified with the
chaetetids, sphinctozoans and stromatoporoids if found as fossils. However, with the
living material in hand, histological, cytological, and larval characteristics can be
observed. This information suggests that these 15 species can readily be placed within
the
Calcarea and the
Demospongia.
This radically changes our view of poriferan phylogeny.

It is widely accepted among poriferan biologists that the Calcarea and the Demospongia are more closely related to each other than either is to the
Hexactinellida. With the discovery of living
chaetetids, stromatoporoids, and sphinctozoans, a fourth class was erected for these
so-called sclerosponges. However, the Sclerospongia is not a natural monophyletic
grouping and is thus being abandoned. The abundant fossil chaetetids,
stromatoporoids, and sphinctozoans are probably part of the classes Demospongia and
Calcarea, though some uncertainty still remains. The Archaeocyatha pose a special
case. No living representative of this group has been discovered. Their organization is
consistent with that of living sponges. The one phylogenetic analysis (carried out by
Reitner) that included archaeocyaths with other sponges, grouped them as sisters
to the demosponges. Therefore, although the taxonomic term Archaeocyatha is often
accorded phylum status it is likely a sub-clade of the phylum Porifera, thereby violating
the ranking system.

Reiswig, H. M. and G. O. Mackie. 1983. Studies on hexactinellid sponges. III. The taxonomic status of Hexactinellida within the Porifera. Philosophical Transactions of the Royal Society of London. B 301: 419-428.

Reitner, J. 1990. Polyphyletic origin of the "Sphinctozoans". In Rutzler, K. (ed.), New Perspectives in Sponge Biology, Proceedings of the Third International Conference on the Biology of Sponges (Woods Hole). pp. 33-42. Smithsonian Institution Press, Washington, DC.